JP4020225B2 - Near-field optical probe - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention uses near-field light to detect the interaction with a minute region of the solid surface, thereby observing structural information or optical information in the minute region below the wavelength of the input light, and high-resolution of the solid surface. The present invention relates to a near-field optical probe that is used for observation or for high-density information recording and reproduction.
[0002]
[Prior art]
High-resolution probes that use near-field light are used in near-field light microscopes and near-field light heads. By generating near-field light from the tip of the probe and detecting propagating light generated as a result of the interaction between the microscope sample or the recording medium and the near-field light, a spatial resolution exceeding the light diffraction limit can be obtained. There is also a method in which near-field light generated as a result of interaction between incident propagation light and a sample or a recording medium is detected by a probe. The near-field microscope achieves a resolution exceeding the diffraction limit of the conventional optical microscope by this principle. Further, when such a near-field optical probe is used for a near-field optical head, a data recording density exceeding that of a conventional optical disk is possible.
[0003]
As a method of using near-field light in a near-field microscope, the distance between the probe micro-aperture and the sample surface is brought close to the diameter of the probe micro-aperture and propagates through the probe toward the probe micro-aperture. There is a method (illumination mode) in which near-field light is generated in the minute aperture by introducing light. In this case, the scattered light generated by the interaction between the generated near-field light and the sample surface is detected by the scattered light detection system with the intensity and phase reflecting the fine structure of the sample surface. It enables observation with high resolution that could not be realized.
[0004]
In addition, an information recording / reproducing apparatus has been studied as an apparatus using near-field light described above.
Many of the current information reproducing apparatuses perform information reproduction for a magnetic disk or an optical disk as an information recording medium. In particular, a CD, which is one of the optical disks, has high-density information recording and low-cost mass production. Therefore, it is widely used as a medium for recording a large amount of information. The CD has a pit having a size of about the wavelength of the laser beam used for reproduction and a depth of about a quarter of the wavelength on the surface of the CD, and utilizes the light interference phenomenon. Reading is possible.
[0005]
In order to read recorded information from an optical disk represented by this CD, a lens optical system generally used in an optical microscope is used. Therefore, when the information recording density is increased by reducing the pit size or the track pitch, the spot size of the laser beam cannot be reduced to a half wavelength or less due to the problem of light diffraction limit. Will hit a wall that cannot be made smaller than the wavelength of the laser beam.
[0006]
Further, not only optical discs but also magneto-optical discs in which information is recorded by a magneto-optical recording method and a phase change recording method, because high-density information recording / reproducing is realized by a minute spot of laser light. The information recording density is limited to the spot diameter obtained by condensing the laser beam.
Therefore, in order to overcome these limitations due to the diffraction limit, an optical head provided with a minute aperture having a diameter equal to or smaller than the wavelength of the laser beam used for reproduction, for example, about 1/10 of the wavelength, is generated at the minute aperture. An information reproducing apparatus using near-field light has been proposed.
[0007]
[Problems to be solved by the invention]
When a near-field optical probe is applied to a microscope, its resolution depends on the probe aperture size, and when it is applied to an information processing apparatus, its information recording density. The principle of achieving resolution or recording density that exceeds the diffraction limit of light with a near-field optical probe is that the aperture size is smaller than the wavelength of the light and has a spatially high frequency in the optical field generated at the tip of the aperture Contains components (components that differ greatly in the direction and intensity of the light field with respect to slight differences in position) and are scattered and detected as propagating light as a result of interaction with the sample or recording medium. That's it. Here, since the resolution or recording density is improved by increasing the high frequency component in the generated light field, efforts have been made to reduce the aperture size.
[0008]
However, the opening size is several tens to several hundreds of nanometers, and it is difficult to further reduce the size. This is because metal film deposition to prevent leakage light has a position control accuracy of about several tens of nanometers, which is the size of the metal cluster. ), It is difficult to make the opening size smaller than the current size.
[0009]
In view of the above problems, the present invention increases the high-frequency component of near-field light without reducing the aperture size, thereby realizing a near-field for realizing a high-resolution microscope or an information processing apparatus for high-density recording. The object is to provide an optical probe.
[0010]
In order to achieve the above object, a near-field optical probe according to a first aspect of the present invention is configured to detect near-field light with a probe provided with a minute aperture for generating near-field light or scattering and detecting near-field light. In the near-field optical probe that causes near-field interaction, Mouth Or the minute opening With mouth Close to the minute aperture Size of It is characterized in that a finer structure is formed.
[0011]
According to the present invention, without reducing the size of the aperture itself, the high frequency component of the light field distribution on the aperture surface is increased, and the spatial high frequency component of the generated near-field light is increased, resulting in observation. By increasing the component containing minute structural information in the light intensity to be obtained, a near-field optical probe for a high-resolution microscope or a high-density recording information processing apparatus can be realized.
[0012]
The near-field optical probe according to claim 2 is the micro structure according to the invention of claim 1. By forming a predetermined of Consists of light intensity distribution Optical property distribution Have It is characterized by that.
According to the present invention, it is possible to form the microstructure by changing the optical characteristic distribution regardless of the size of the opening, and it is easy and inexpensive to use for a high-resolution microscope or a high-density recording information processing apparatus. A field optical probe can be realized.
[0013]
A near-field optical probe according to claim 3 is the invention according to claim 2, wherein The minute structure is formed inside the minute opening and in the vicinity of the opening end. Uneven shape A groove structure having It is characterized by that.
According to the present invention, since it is a change that only processes the shape of the tip of the optical probe.
New effects can be obtained without significantly changing the manufacturing process.
[0014]
A near-field optical probe according to claim 4 is the invention according to claim 2, wherein The minute structure is formed inside the minute opening and in the vicinity of the opening end. Material distribution Is made of a material distribution structure formed so as to have a predetermined distribution It is characterized by that.
According to the present invention, it is easy to select and control the material to be injected into the opening and the particle size thereof, and a desired resolution or recording density can be obtained.
[0015]
In a near-field optical probe according to a fifth aspect of the present invention, in the invention according to any one of the first to fourth aspects, the minute opening is a bottom of an inverted conical hole formed through the flat substrate. Features.
According to the present invention, since the probe is flat, a more compact device configuration is achieved. Furthermore, since the planar probe can be produced using semiconductor manufacturing technology, it can be mass-produced with high reproducibility, and when it is used in an information processing apparatus, it is used in a conventional hard disk. A head floating mechanism such as the flying head method can be used as it is.
[0016]
According to a sixth aspect of the present invention, in the near-field optical probe according to any one of the first to fourth aspects, the minute opening is formed at the tip of a sharpened optical waveguide. .
According to the present invention, since the optical fiber type probe used in the conventional near-field microscope can be used as the probe, the accumulated near-field microscope technology can be effectively applied. Moreover, since it is also possible to manufacture a probe using a semiconductor manufacturing technique, the accumulated semiconductor manufacturing technique can be applied effectively.
[0017]
According to a seventh aspect of the present invention, in the near-field optical probe according to any one of the first to fourth aspects, the minute opening is formed in the protrusion of the cantilever in which a sharp protrusion is formed. It is characterized by.
According to the present invention, since the cantilever type probe used in the conventional near-field microscope can be used as the probe, the accumulated near-field microscope technique can be effectively applied.
[0018]
DETAILED DESCRIPTION OF THE INVENTION
Embodiments of a near-field optical probe according to the present invention will be described below in detail with reference to the drawings.
(Embodiment 1)
FIG. 1 is a block diagram showing a schematic configuration of an information reproducing apparatus using an optical probe according to Embodiment 1 of the present invention. In FIG. 1, the information reproducing apparatus using the optical probe according to the first embodiment includes a near-field optical probe 1 that generates near-field light 6, an information recording medium 3 on which data marks 2 are formed at high density, and data A light receiving element 4 that receives propagating light 7 scattered by the mark 2 and outputs an electric signal, and a signal processing circuit 5 that amplifies the electric signal output from the light receiving element 4 and outputs the amplified signal.
[0019]
FIG. 2 is a diagram for explaining the near-field optical probe 1 in detail. The near-field optical probe 1 is formed by heating, stretching, cutting, and Al coating of an optical fiber, and includes a core portion 8 and a light shielding film 9. In the distal end surface 10 of the core portion 8, there are dotted portions 11 that are recessed from the surface. Such a probe having a tip is prepared by chemical etching by RIE (Reactive Ion Etching reactive ion etching) or irradiation of the tip surface of Ar or Cl by milling. In milling, an optical fiber or an optical probe is arranged vertically with the tip surface facing up, and ions are irradiated from above. In this embodiment, the tip of the optical fiber is 10 ^-6 It was placed in a Torr vacuum and irradiated with Ar + ions for 10 minutes at an acceleration and density of 4.0 kV, 1 mA. As a result, an unevenness of several tens of nanometers was formed on the fiber end face.
[0020]
As a result, the light intensity distribution 12 in the tip surface 10 has a distorted shape rather than the conventional rectangular function. According to Fourier optics, the aperture can be represented by a superposition of amplitude diffraction gratings having different lattice constants. An aperture with a finite extent must always be from a lattice with a lattice constant longer than the wavelength, because of the analytical connectivity of the finite function.
It can be expanded to a lattice having a lattice constant sufficiently shorter than the wavelength. Among these, a lattice constant component shorter than the wavelength, that is, a component having a high spatial frequency creates near-field light. At this time, in the case of the present embodiment in which the light intensity 12 in the aperture has a form of a spatially disturbed function instead of a rectangular function, the Fourier expansion has a component with a high spatial frequency.
[0021]
Propagating light 7 generated as a result of the interaction of the near-field light with the data mark 2 in the recording medium 3 is a near-field light having a large spatial frequency component corresponding to the data mark 2 even if the data mark 2 is smaller than the aperture size. Identified by interaction with.
In the present embodiment, the resolution of the probe before forming the microstructure by milling in the opening is measured and compared with the resolution after forming the microstructure. As a result, the resolution of the probe having no microstructure in the aperture plane was 200 nm, whereas the resolution of the probe having the microstructure was improved to 100 nm.
[0022]
Thus, it is possible to increase the resolution of the probe without reducing the aperture size, and it has been shown that high-density information reproduction can be performed by using this probe for an information reproducing apparatus.
In this embodiment, a microstructure is formed in the opening surface, but the edge of the opening is not formed in the conventional circular shape by the same ion milling method, but is formed into a shape in which a minute uneven structure of several tens of nanometers is formed. The same effect can be obtained by.
[0023]
(Embodiment 2)
FIG. 3 is a block diagram showing a schematic configuration of a near-field optical microscope using the near-field optical probe according to Embodiment 2 of the present invention. In FIG. 3, a laser light source 13, a condensing lens 14, a mirror 15, and a photoelectric conversion element 16 that is divided into two parts are installed above the probe 19, and light emitted from the laser light source 13 is transmitted by the condensing lens 14. The light collected on the probe upper surface 20 and reflected there is introduced into the photoelectric conversion element 16 via the mirror 15. The light emitted from the light source 24 for measuring optical information is irradiated from the back surface to the sample 21 on the prism 22 whose total reflection processing is applied to the inclined surface through the collimator lens 25, and the probe 19 close to the sample 21 is irradiated. Is introduced into the photoelectric conversion element 17.
[0024]
The prism 22 and the sample 21 are installed on a coarse movement mechanism 27 and a fine movement mechanism 26 that can move in the xyz direction. The signal detected by the photoelectric conversion element 16 is sent to the servo mechanism 23. Based on this signal, the servo mechanism 23 controls the coarse movement mechanism 27 and the fine movement mechanism 26 so that the deflection of the probe 19 does not exceed the specified value when the probe 19 approaches the surface of the sample 21 or the surface is observed. It has become. A computer 29 is connected to the servo mechanism 23, and controls the operation of the fine movement mechanism 26 in the planar direction and receives surface shape information from the control signal of the servo mechanism 23. If the signal from the photoelectric conversion element 17 modulates the light from the light source 24, or if vibration is applied between the probe 19 and the sample 21, the analog input interface of the computer 29 via the lock-in amplifier 28 The optical information is detected in synchronization with the planar operation of the fine movement mechanism 26. When the light source 24 is not modulated or the like, the signal of the photoelectric conversion element 17 is directly connected to the analog input interface of the computer 29 without passing through the lock-in amplifier 28.
[0025]
Since the optical probe used in the present embodiment is the same as that used in the first embodiment, a detailed description of the structure and production method is omitted.
In such an optical probe, the optical characteristic (refractive index) in the tip opening surface is not flat but has a changed distribution. That is, the size of the opening is the same as that of the conventional optical probe, but optically, it has the same effect as a finer structure than the opening is formed in the opening. Near-field light generated on the surface of the sample 21 includes fine structural information on the surface of the sample 21 in its intensity distribution. When the optical probe interacts with the near-field light, the shape of the distribution of optical characteristics in the tip opening surface is smaller than the aperture size, so it interacts with the near-field light including the fine structure information on the surface of the sample 21. Occurs and propagates into the optical probe to be detected as propagating light.
[0026]
In the present embodiment, the resolution of a near-field optical microscope using an optical probe whose tip is not machined is measured as in the first embodiment, and then the tip is machined and the resolution is similarly measured and compared. did. As a result, the microscope with the optical probe before processing the tip had a resolution of about 200 nm, whereas the microscope with the optical probe after processing had a resolution of about 100 nm.
[0027]
In this way, the resolution of the near-field light microscope could be increased without reducing the aperture size.
(Embodiment 3)
FIG. 4 is a diagram for explaining a third embodiment in which a near-field optical probe is formed into a planar type using a semiconductor manufacturing technique and used for a high-density information recording / reproducing apparatus. More specifically, the posture of accessing the recording medium is shown together with the cross-sectional structure of the recording medium. The slider 30 is supported by a suspension arm (not shown). The suspension arm and the slider 30 constitute a flying head mechanism. The suspension arm swings around a swing shaft using a voice coil motor (not shown) as a drive source. A taper 33 is provided in the scanning direction of the slider 30. The taper 33, the slider bottom surface 38, and the surface of the recording medium 35 form a wedge film-shaped air flow path 34. The slider 30 is given load load to the recording medium 35 side by a suspension arm and a gimbal spring. The slider 30 is positioned on the track of the recording medium 35 by seek control and following control. By rotating the recording medium, the slider scans relative to the recording medium. The slider 30 has an inverted conical hole formed as a light passage 32. The light passage 32 is filled with a glass material that transmits visible light. The tip of the light path 32 is a minute opening 39 on the bottom surface of the slider 30, and the opposite side (the top surface of the inverted conical hole) is covered with a light emitting element 31 bonded to the top surface of the slider 30. A minute uneven structure is formed in the minute opening 39 by the same ion milling process as described in the first embodiment. A data mark 36 for storing unit data is formed on the recording medium 35.
[0028]
The slider having such a structure is produced by a semiconductor micromachining technique such as anisotropic etching. The light generated by the light emitting element 31 is guided to the minute opening 39 through the light passage 32. Here, since the minute aperture 39 is smaller than the wavelength of light, a light field mainly composed of near-field light 37 is formed on the recording medium 35 side of the minute aperture 39. Data is recorded / read by the interaction between the near-field light 37 and the data mark 36. Since the output signal obtained in time series is information on the presence / absence of the data mark at each time, the position information of the data mark on the recording medium can be obtained from this and the rotational speed of the recording medium.
[0029]
Similar to the first and second embodiments, the light distribution in this surface is not a rectangular function but is disturbed by the fine uneven structure on the minute opening surface, and therefore includes spatial high-frequency components. It is out. This is the same as FIG. 2 in the first embodiment. The near-field corresponding to this high-frequency component, that is, a short wavelength, allows the data mark to be identified by the interaction with near-field light having a large wavelength component even if the data mark 36 is smaller than the aperture size. High-density information recording / reproduction is possible.
[0030]
(Embodiment 4)
In the present embodiment, as the near-field optical probe 1 implemented in the first embodiment, a cantilever with an aperture processed with SiN is used instead of an optical fiber. The cantilever is generally used in an atomic force microscope (AFM), and is produced using a generally known three-dimensional microfabrication technique for a wafer. After that, Al was deposited on the portion other than the sharp tip so that the tip became an opening. A fine structure was formed on the opening surface by ion milling performed in Embodiment 1 for this opening. As a result, the light distribution on the aperture surface contains a large amount of high spatial frequency components, that is, short wavelength components, and high-density information recording / reproduction was realized without reducing the aperture size.
[0031]
Further, by using a cantilever probe instead of an optical fiber as an optical probe, a probe having a small spring constant and a large resonance frequency could be produced. The probe having a small spring constant enables scanning with a small interaction between the probe and the surface of the recording medium, and damage to the probe tip and the surface of the recording medium is reduced. In addition, the probe having a large resonance frequency makes it easier for the probe to follow the irregularities on the surface of the recording medium, thereby enabling faster scanning.
[0032]
(Embodiment 5)
FIG. 5 shows the tip structure of the near-field optical probe in the fifth embodiment. The optical probe was sharpened by heating and stretching the tip of the optical fiber, and made with Al coating. The core portion 41 of the fiber is covered with the Al film 40 at the tip. A styrene molecular assembly 42 is chemically bonded to the end face of the core portion by silane coupling. In the silane coupling, an organic molecule is bonded to a dangling bond of Si atoms on the probe tip surface to form a structure as shown in FIG. The 42 layers of styrene molecular aggregates are aggregates of styrene molecules without having a specific ordered structure. If the entire aggregate is considered as one structure, the optical properties (refractive index) in it have a complex distribution. Will have. As a result, the light intensity at the aperture plane is not a conventional rectangular function, but includes spatially high frequency components as shown in FIG. As described in the second embodiment, the resolution of the microscope using the near-field optical probe is high-performance by using the short-wavelength component of the near-field, that is, the high-frequency component. By making the distribution spatially complex, a high-resolution near-field microscope was obtained.
[0033]
Furthermore, as an advantage of using silane coupling, the light distribution on the aperture can be easily changed by chemically modifying organic molecules coupled to the aperture or changing the molecular length. In this embodiment, the styrene molecular assembly 42 is coupled to the opening surface, but such an assembly is formed inside several hundred nanometers from the opening surface, and the bottom is the same as the core of the optical fiber. The same effect can be obtained by coating with a material.
[0034]
(Embodiment 6)
In the present embodiment, the near-field optical probe created in the fifth embodiment is used for the high-density information recording / reproducing apparatus implemented in the first embodiment. The structure and production method of the near-field optical probe are the same as those in the fifth embodiment. The structure and operation method of the high-density information recording / reproducing apparatus are the same as those in the first embodiment. The light intensity at the opening surface at the tip of the near-field optical probe is not a conventional rectangular function, but includes spatially high frequency components as shown in FIG. As described in the first embodiment, the information recording density can be increased without reducing the aperture size by using the high-frequency component in the near field.
[0035]
(Embodiment 7)
FIG. 6 shows the tip structure of the near-field optical probe in the seventh embodiment. The difference from FIG. 5 is that the present embodiment has a structure in which boron ion clusters 43 are embedded in the tip of the optical fiber core 41 by ion plantation. The cluster of boron ions has an average particle size of about 10 nm, and the optical properties (refractive index) of the tip surface are not uniform and have an in-plane distribution, so that the light intensity distribution is a conventional rectangle function. Instead, it contains spatially high frequency components as shown in FIG. This optical probe was used in the information recording / reproducing apparatus implemented in the first embodiment. As described in the first embodiment, the information recording density can be increased without reducing the aperture size by using the high-frequency component in the near field. In this embodiment, the boron ion cluster is embedded in the tip surface of the optical probe, but the same effect can be obtained even if this cluster is embedded several hundred nanometers from the tip surface.
[0036]
(Embodiment 8)
In the present embodiment, the near-field optical probe created in the eighth embodiment is used for the near-field light microscope implemented in the second embodiment. As described in the second embodiment, the resolution of the microscope using the near-field optical probe is high-performance by using the short-wavelength component of the near-field, that is, the high-frequency component. By making the distribution spatially complex, a high-resolution near-field microscope was obtained.
[0037]
【The invention's effect】
As described above, according to the first aspect of the present invention, since a minute structure is formed on the minute opening surface or in the vicinity of the minute opening surface, the light amount can be reduced without reducing the opening size. It can be used for high-resolution near-field microscopes or high-density information recording / reproducing devices that are easily and inexpensively manufactured with high reproducibility and yield, and that can easily control resolution. Play.
[0038]
According to the invention of claim 2, since the minute structure is due to the optical characteristic distribution in the minute aperture surface or in the vicinity of the minute aperture surface, in addition to the effect of claim 1, the size of the aperture Regardless of this, it is possible to form the microstructure by changing the optical characteristic distribution, and it is possible to realize a near-field optical probe for a high-resolution microscope or a high-density recording information processing apparatus easily and inexpensively. .
[0039]
Further, according to the invention according to claim 3, since the optical characteristic distribution is due to the concavo-convex shape near the opening surface or the opening surface, it is a change only by processing the shape of the tip of the optical probe. There is an effect that a new effect can be obtained without greatly changing the manufacturing process.
According to the invention of claim 4, since the optical characteristic distribution is due to the distribution of the material at or near the aperture surface, the material to be injected into the aperture and the particle size thereof are selected and controlled. It is easy to perform, and there is an effect that a desired resolution or recording density can be obtained.
[0040]
According to the invention of claim 5, since the minute opening is the bottom of an inverted conical hole formed through the flat substrate, a more compact device configuration is achieved. Furthermore, since the planar probe can be produced using semiconductor manufacturing technology, it can be mass-produced with high reproducibility and can be obtained at a relatively low cost. Further, when this is used in an information processing apparatus, there is an effect that a head floating mechanism such as a flying head method used in a conventional hard disk can be used as it is.
[0041]
According to the invention of claim 6, since the minute opening is formed at the tip of a sharpened optical waveguide, an optical fiber type probe used in a conventional near-field microscope is used. It is possible to use the accumulated near-field light microscope technology effectively. In addition, since it is possible to create a probe using a semiconductor manufacturing technique, the accumulated semiconductor manufacturing technique can be effectively applied.
[0042]
Further, according to the invention of claim 7, since the minute opening is formed in the protruding portion of the cantilever in which a sharp protrusion is formed, the cantilever type used in the conventional near-field microscope Thus, the accumulated near-field optical microscope technology can be effectively applied.
[Brief description of the drawings]
FIG. 1 is a block diagram showing a schematic configuration of an information reproducing apparatus using an optical probe according to a first embodiment.
FIG. 2 is a diagram for explaining the near-field optical probe 1 in detail.
3 is a block diagram showing a schematic configuration of a near-field light microscope using a near-field optical probe according to Embodiment 2. FIG.
FIG. 4 is a diagram for explaining a third embodiment in which a near-field optical probe is formed into a planar type using a semiconductor manufacturing technique and used in a high-density information recording / reproducing apparatus.
FIG. 5 is a diagram showing a tip structure of a near-field optical probe in a fifth embodiment.
FIG. 6 is a diagram showing a tip structure of a near-field optical probe in a seventh embodiment.
[Explanation of symbols]
1 Near-field optical probe
2 Data mark
3 Information recording media
4 Light receiving element
5 Signal processing circuit
6 Near-field light
7 scattered light
8 Core part
9 Shading film
10 Core tip
11 Recessed part from the core tip
12 Light intensity distribution at the tip of the core
13 Laser light source
14 Condensing lens
15 Mirror
16, 17 Photoelectric conversion element
18 Probe support
19 Probe
20 Probe upper surface
21 samples
22 Prism
23 Servo mechanism
24 Light source
25 Collimating lens
26 Fine movement mechanism
27 Coarse motion mechanism
28 Lock-in amplifier
29 computers
30 slider
31 Light emitting device
32 Light passage
33 Taper
34 Air flow path
35 Recording media
36 Data mark
37 Near-field light
38 Slider bottom
39 Small aperture
40 Al film
41 core
42 Styrene molecular assembly
43 Boron cluster

Claims (6)

In a near-field optical probe that generates near-field light or causes near-field interaction with an object by a probe provided with a minute aperture for scattering and detecting near-field light,
The material in the minute opening is formed with a structure smaller than the size of the minute opening ,
The near-field optical probe characterized in that the minute structure has an optical characteristic distribution consisting of a predetermined light intensity distribution . The near-field optical probe according to claim 1 , wherein the minute structure is a groove structure having an uneven shape. 2. The near-field optical probe according to claim 1 , wherein the minute structure includes a material distribution structure formed so that the material distribution is a predetermined distribution. The microscopic aperture is, near-field optical probe according to any one of claims 1-3, characterized in that the bottom portion of the reverse conical hole formed through the planar substrate. The microscopic aperture is, near-field optical probe according to any one of claims 1-3, characterized in that the one formed at the tip of the sharpened optical waveguide. The microscopic aperture is, near-field optical probe according to any one of claims 1-3, characterized in that in which sharp projections are formed on the protrusion of the cantilever formed.

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